This invention relates generally to the field of superconducting ceramics and particularly to manufacturing methods for making superconducting ceramics.
It has long been known. that metals such as mercury and lead, intermetallics such as NbNd, Nb3Ge and Nb3Ga and ternary materials such as Nb3(Al0.8Ge0.2) demonstrate superconductivity. However, the transition temperature of such long known superconducting materials cannot exceed 25xc2x0 K.
In more recent years, superconducting ceramics have attracted widespread interest. A new material was first reported by researchers at the Zurich laboratory of IBM Corp. as Ba-La-Cu-0-type high temperature superconducting oxides. Subsequently, La-Sr-Cu(II)-0-type superconducting oxides were also proposed. Another type of superconducting material that has been found is (YBa2)Cu3O6-8. By virtue of the fact that these superconducting ceramics form a quasi-molecular atomic unit in a crystalline structure whose unit cell is constructed with one layer in which electrons have essentially one-dimensional motion, whereas only three-dimensional electron conduction occurs in the long known materials mentioned above, higher transition temperatures are achieved.
Much work has been undertaken by researches in this field who have endeavoured to elevate Tco, the temperature at which resistance vanishes, above the levels previously obtained and preferably above the boiling point of nitrogen (77xc2x0 K.) or even higher. As described in our European Patent Application No. 87309081.5 we have investigated superconducting ceramics materials having the stoichiometric formulae (A1xe2x88x92xBx)yCuzOw, where A represents one or more elements of Group IIIb of the Periodic Table, e.g. the rare earth elements, and B represents one or more elements of Group IIa of the Periodic Table, e.g. the alkaline earth elements including beryllium and magnesium, and in the continuation of these investigations we have discovered that the existence of voids and grain boundaries in superconducting ceramic materials makes it difficult to obtain an elevated Tcc.
The present invention thus seeks to provide superconducting ceramics having a higher transition temperature than hitherto and to manufacture superconducting ceramics substantially devoid of imperfections.
Whereas in previous attempts to find higher Tc superconducting materials attention has been focussed upon the composition or molar ratios of the constituent elements, in accordance with the present invention there is principally provided an improved process for the manufacture of superconducting ceramics in accordance with which, during the firing of the raw ceramic constituents, that is to say the chemical compounds which are mixed together to form the superconducting ceramic, an electric current is passed therethrough. By virtue of the passage of this current, it has been found that the ceramic mixture can be given a special orientation in which the atomic arrangement is ordered and made more simple, e.g on the (a,b) plane, so that few grain boundaries and imperfections exist in the final ceramic material.
Whilst the exact mechanism whereby this advantageous effect is obtained is not fully understood, one possibility is that, analogous to the molecular motion which occurs in a magnetic substance in response to the application of a magnetic field thereto, the dipole moments in superconducting ceramics materials may be oriented by the application thereto of an electric field so as to result in a well-arranged structure.
Other features of the invention are set forth with particularity in the appended claims and will become clear to those possessed of the relevant skills from consideration of the following description of exemplary embodiments given with reference to the accompanying drawings.